This invention relates to the offshore resource-recovery devices and processes. More specifically, the invention is concerned with improved oil and gas or other multi-phase fluid production from offshore subsea wells, especially from ultra-deep offshore wells.
Some offshore resource recovery activities, e.g., withdrawal of hydrocarbon fluids from a subsurface reservoir through a well tubular and riser assembly to surface fluid delivery facilities, have previously been accomplished using an offshore platform. The offshore platform typically supports at least a portion of the riser and fluid delivery facilities and other equipment needed to process and recover resource fluids.
For shallow water depth locations, a well and fluid delivery system typically includes a riser and the remainder of the fluid delivery system that is generally located on a rigid platform structure fixed to a seafloor anchor or foundation. For deepwater offshore platforms locations, e.g., offshore platforms located in waters having a depth exceeding about 1,500 feet (or about 457 meters), this type of fixed tower structure is typically not cost effective, and other types of facilities may be used, e.g., subsea wellheads and delivery systems.
As the distance between the subsea wellheads and surface processing facilities increases, e.g., due to increasing water depths, the addition of external energy to the recovered fluids may become necessary to recover commercial quantities of oil or other fluids from deepwater reservoirs. For wells in deepwater locations, especially in ultra deepwater locations (herein defined as water surface to mudline depths of at least about 10,000 feet or 3,000 meters), the addition of external energy may extend the working range of reservoir pressures that can be produced. The additional external energy can be a major factor in producing commercial flowrates of oil and gas from these deepwater or ultra-deepwater resources.
One of the items of equipment that may be required to process and recover commercial quantities of oil and/or gas from deep, multi-phase reservoirs is a pump. The pump must be able to handle multi-phase fluids such as oil with lighter hydrocarbon or inert gases, oil with steam or flashing hot brine, slurries, or other fluid-like mixtures of components having density differences.
One embodiment of the inventive fluid delivery system comprises a vertical, low-pressure fluid separator and integral vapor riser assembly having a liquid outlet port connected to a pump assembly, preferably hydraulically driven. The pump assembly increases the pressure of the separated liquid allowing the delivery of pressurized liquid to other fluid handling facilities at the surface. The pump speed is simultaneously controlled to limit the range of vapor/liquid interface levels within the separator. The large variation in liquid interface levels within the vertical separator also allows the use of a subsea hydraulically-driven pump (typically having a relatively slow reaction time especially if hydraulically driven from a surface source of pressurized fluid) even during periods of system upsets. Because of the system upset tolerance, the relatively open system design, and simplicity of the operating controls, the present invention is expected to be reliable, safe, and cost effective. Moreover, the removal of most of the liquid-phase from the vapor riser allows a minimum operating or reservoir abandonment pressure, minimizing the backpressure on the subsea well.